An investigation of some factors influencing the rate of oxidation of elemental sulphur fertilizers : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University
Methodologies for measuring the particle size of So in pure and compound fertilizers (sulphurized superphosphates (SSP), reactive phosphate rocks (RPR) and partially acidulated phosphate rock (PAPR)) and for determining the rate at which So in these materials oxidises in soils were evaluated and improved. Sample dispersion in 10% HC1 followed by wet sieving was the most successful method for sizing So in SSP, RPR and PAPR based fertilizers. So/bentonite fertilizers, however, were more easily dispersed in water than in acid. Acetone extraction (40 g:200 ml acetone, using a 16 h shaking period) and determination of So in the extract proved to be a suitable method for measuring amounts of So in finely ground fertilizers and soils at concentrations above 5 μg S g-1 soil and below 200 μg S ml-1 acetone. The rate of So oxidation in soil was determined by regularly measuring residual amounts of So. The influence of soil type and fertilizer history on the potential of soils to oxidise So was examined in incubation studies. On average, soils that had previously received So applications had higher initial rhodanese enzyme activities (RA) and higher So oxidation rates but there was no simple relationship between fertilizer history or RA and initial So oxidation rate. Different sources of So, namely Rotokawa So (geothermal So), dark So, Damman So, and agricultural grade So had similar oxidation rates per unit surface area. Granules or prills oxidised slowly in incubated soil because they did not disintegrate when placed in soil and had small specific surface area. On average, the oxidation rate of So was increased when mixed or granulated with reactive phosphate rocks and incorporated in soil but this effect was not consistently reproducible. Further incubations of So in the presence of various combinations, CaHPO4, CaCl2 and CaCO3, demonstrated that the presence of CaHPO4 and CaCO3 could elevate So oxidation rates. Granulation of RPR and PAPR with So did not significantly increase (p >0.05) the oxidation rate of So surface applied to undisturbed pasture soils (glasshouse and field trials). Under surface application conditions granulated So had similar oxidation rates to finely divided So forms. An iterative computer program was developed to calculate specific oxidation rates (K, μg So cm-2 day-1) from the amounts of acetone extractable So remaining in soils at different times. On average, K for <150 So μm was significantly lower (p <0.05) when surface applied to undisturbed soil cores than when incorporated into incubated soils. Specific oxidation rates of different particle sizes (<150, 150-250 and 250-500 μm) of surface applied So were similar (ranging form 11-19 μg So cm-2 day-1) but were different (P <0.05) for the two soil types used in glasshouse trials (means of 17 and 13 μg So cm-2 day-1 for Ramiha and Tokomaru soil, respectively). Corrections for the effects of soil moisture on oxidation rates provided evidence that all So could have similar maximum potential K values (Kmax = 18 μg S cm-2 day-1) in both soils. This suggested, with other evidence from the literature, that So oxidation in soil could be effectively modelled by knowing So particle size and the effects of soil moisture and temperature on So oxidation. A So oxidation simulation model was constructed using a value for Kmax determined in the glasshouse trials. Within experimental error, the simulation model predicted So oxidation in field soil well (explaining between 76 and 97% of data variance at 3 field sites) and provides a useful basis for designing future research projects.